In cellular mobile radio systems, it is fundamental that a mobile station with an established connection on a radio channel shall be able to maintain the established connection when moving from one cell serviced by a base station to another cell serviced by another base station. It is also highly desirable that the mobile station with an established connection on a radio channel shall be able to maintain the established connection when moving within the same cell or when the radio channel which is used is subject to increased interference. The process by which a mobile station can maintain an established connection when moving in a cellular mobile radio system is generally called a handoff.
In general, radio communication is only possible when the desired information carrying radio signals have sufficient signal strength at the receiver and are sufficiently strong relative to the noise and interfering radio signals at the receiver. The minimum strength, of course, depends on the particular features of the system, e.g., the kind of modulation and the type of receiver. In order to insure that the established connection may continue on an intended radio channel between a mobile station and an intended base station, the handoff procedure includes measurements of the parameters of the radio signals at the intended base station and/or at the mobile station.
The first cellular mobile radio systems placed in public use were analog systems typically used for speech or other types of analog information. These systems include multiple radio channels for transmitting analog information between bases and mobile stations by transmitting analog modulated radio signals. In general, the first cellular mobile radio systems had relatively large cells, and the signal measurements in the handoff process in such systems were performed by the base station. One such system is the Nordic Mobile Telephone System NMT-450. Another known cellular mobile radio system is the AMPS Mobile Radio System in the United States. The general description of a mobile cellular radio system can be found in a publication entitled "CMS 88 Cellular Mobile Telephone System" published by Ericsson Telecom AB, 1988.
Currently, channel access is achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. As illustrated in FIG. 1(a), in FDMA, a communication channel is a single radio frequency band into which a signal's (S1, S2) transmission power is concentrated. Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel reuse.
In TDMA systems, as illustrated in FIG. 1(b), a channel consists of a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal's (S1, S2, S3, S4, S5) energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that possess signal energy received at the proper time. Thus, the problem of interference from different relative signal strength levels is reduced.
Capacity in a TDMA system is increased by compressing the transmission signal into a shorter time slot. As a result, the information must be transmitted at a correspondingly faster burst rate which increases the amount of occupied spectrum proportionally.
With FDMA or TDMA systems or hybrid FDMA/TDMA systems, the goal is to insure that two potentially interfering signals do not occupy the same frequency at the same time.
In contrast, Code Division Multiple Access (CDMA) allows signals to overlap in both time and frequency, as illustrated in FIG. 1(c). Thus, all CDMA signals share the same frequency spectrum. In either the frequency or the time domain, the multiple access signals (S1, S2, S3, S4) appear to be on top of each other. In principle, the informational data stream to be transmitted is impressed upon a much higher bit-rate data stream generated by a pseudo-random code generator. The informational data stream and the high bit-rate data stream are combined by multiplying the two bit streams together. This combination of the higher bit-rate signal with the lower bit-rate data stream is called coding or spreading the informational data stream signal. Each informational data stream or channel is allocated a unique spreading code. A plurality of coded information signals are transmitted on radio frequency carrier waves and jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise related signals, in both frequency and time. By correlating the composite signal with one of the unique codes, the corresponding information signal is isolated and decoded.
In an attempt to give cellular systems more capacity, voice channels may be added to a cell in the existing radio frequency band (primary frequency band) or added to the cell in a new radio frequency band (extended frequency band). In the EIA/TIA IS-54 standard, the extended frequency band may be located at frequencies located above and/or below the frequencies contained in the primary frequency band. In one embodiment of the present invention, one or more of the base stations in the communication system may contain channels in both the primary frequency band and the extended frequency band.
In some cellular mobile systems, several different groups of channels are simultaneously available. As a result, there are three kinds of mobile stations which may be operating in the communication system. The first kind of mobile station is capable of using only a first group of channels. A second kind of mobile station is capable of using only a second group of channels. A third kind of mobile station is capable of using either the first or second group of channels.